LCD devices are most widely used flat panel displays and used in consumer apparatuses such as a personal computer, a word processor, and a cellular phone. A further expansion of its market is expected. The market expansion generates a need of further improving image qualities of the LCD devices. An LCD device with high display performances such as a wide viewing angle, a high contrast ratio, and a high response time is now being researched and developed.
CPA (continuous pinwheel alignment) mode, which is a kind of VA (vertical alignment) mode, is known as a display mode of LCD devices excellent in viewing angle characteristics and the like (for example, refer to Non-patent Document 1). According to VA mode, negative liquid crystals with negative dielectric anisotropy are vertically aligned to substrate surfaces when no voltage or a voltage not larger than a threshold voltage is applied, and the negative liquid crystals are horizontally aligned thereto when a voltage not smaller than a threshold voltage is applied. According to the CPA mode, continuous pinwheel alignment of negative liquid crystals with negative dielectric anisotropy can be provided in each pixel, only by an electrode shape or a combination of an electrode shape and an alignment control projection. For example, in the case when a plurality of sub-pixels are formed in a pixel displaying a single color of red, green, blue, or the like, continuous pinwheel alignment of liquid crystals in adjacent ones of the sub-pixels can be achieved by providing an electrode with an alignment control projection at a position corresponding to the center of sub-pixels of the opposite electrode or by providing an electrode with openings, thereby forming it into a shape for pinwheel alignment at such a position.
Patent Documents 1 to 3 disclose, for example, an LCD element or a LCD display that provides display by pinwheel alignment of liquid crystal molecules interposed between a pair of substrates. According to Patent Document 1, the LCD element includes: a first electrode on one surface; a second electrode facing the first electrode and having openings for dividing a pixel into a plurality of sub-pixels; a vertical alignment film; a liquid crystal layer with negative dielectric anisotropy, interposed between two substrates; and a storage electrode formed in a periphery region surrounding at least the second electrode. In this case, alignment of liquid crystals is controlled by cross-shaped openings of the second electrode, and continuous pinwheel alignment of the liquid crystals from the periphery to the center of the sub-pixel can be achieved upon application of a voltage.
The LCD device of Patent Document 2 has the following configuration: VA liquid crystal layer is arranged between first and second substrates; the first substrate has a first alignment control structure for forming a plurality of domains where pinwheel alignment is achieved upon application of a voltage in the liquid crystal layer; the second substrate has a second alignment control structure in a region facing at least one of the plurality of domains; the first alignment control structure is composed of at least one opening and the body of an electrode (sub-pixel); and the liquid crystal molecules whose alignment is controlled by the at least one opening and the liquid crystal molecules whose alignment is controlled by the body are aligned in a continuous pattern. In this case, the opening is formed in the first substrate, and the second alignment control structure is formed in the second substrate, thereby achieving continuous pinwheel alignment of the liquid crystals from the periphery of the body toward the center upon application of a voltage. The singular point in the body is fixed by the second alignment control structure.
According to the LCD device of Patent Document 3, two substrates each including an electrode and a vertical alignment film are arranged to face each other with a specific space therebetween, and liquid crystals with negative dielectric anisotropy are injected into the space. The LCD device has a singular point control portion that controls alignment of the liquid crystals so that a singular point of the alignment vector is formed at a specific position upon application of a voltage between the electrodes. Using this singular point, the device achieves alignment control of the liquid crystals. In this case, an oblique electric field is generated by forming a projection or an electrode-free region, and thereby the alignment control and the singular point-control are achieved.
A polymer sustained alignment (hereinafter, also referred, to as “PSA”) technology is known as a technology of improving a response time or an aperture ratio (for example, refer to Patent Documents 4 and 5). According to the PSA technology, monomers are dispersed into liquid crystals, and under application of a voltage to the liquid crystals, the monomers are photopolymerized, and as a result, polymers are formed on an alignment film surface. These polymers can fix a pretilt angle of the liquid crystals on the alignment film surface.    [Patent Document 1]    Japanese Kokai Publication No. 2006-53546    [Patent Document 2]    Japanese Patent No. 3601788    [Patent Document 3]    Japanese Kokai Publication No. 2001-249340    [Patent Document 4]    Japanese Patent No. 3520376    [Patent Document 5]    Japanese Kokai Publication No. Hei-08-114804    [Nonpatent Document 1]    Masumi Kubo and three others,    “Continuous Pinwheel Alignment (CPA) mode wo motiita ASV-LCD no kaihatsu”, Sharp technical report No. 80 on pages 11 to 14, August, 2001.
According to the embodiment of Patent Document 1, the device has no means for controlling a position of a singular point formed at the center of pinwheel alignment. So if the sub-pixel is large, a region where an oblique electrical field is not generated is formed. As a result, the position of the singular point is not stabilized, leading to rough-grained image, generation of image retention, and the like. Thus, the size of the sub-pixel needs to be sufficiently decreased in order to stabilize the position of the singular point. If the size thereof is sufficiently decreased, however, a proportion of an opening in a pixel electrode is increased, resulting in a reduction in transmittance. In such a point, the embodiment of Patent Document 1 has room for improvement.
According to the embodiment of Patent Document 2, the second alignment control structure needs to be formed on the second substrate, which complicates the production steps. The pinwheel alignment and stabilization thereof are both attributed to the same alignment control structure. The device has no structure that can stabilize the pinwheel alignment by itself. So a region where an oblique electric field is not generated is formed in a large sub-pixel in a large-sized display device, resulting in rough-grained image, generation of image retention, and the like. In such a point, the embodiment of Patent Document 2 has room for improvement.
According to the embodiment of Patent Document 3, an oblique electric field is generated by a projection or an electrode-free region, and alignment control and singular point-control are attributed to either one of the two. If the singular point-control portion is pattern-formed to have a large line-width, an oblique electric field is generated in the width direction of the pattern upon application of a voltage. As a result, the liquid crystals are aligned in the width direction of the control pattern. In such a case, the position of the singular point is not stabilized, resulting in flow alignment. This causes rough-grained image or generation of image retention. In such a point, the embodiment of Patent Document 3 has room for improvement. If the line-width is small, an oblique electric field is not so generated and so alignment control force is weak, even if liquid crystals are aligned in an extending direction of the control pattern. For alignment control using only the singular point-control portion, the singular point-control portions need to be closely arranged within the pixel electrode, thereby reducing a space between the singular point-control portions. In this case, the transmittance is reduced. So the embodiment of Patent Document 3 has room for improvement.